1. Field of the Invention
The present invention relates to a squeezing detection control method in consumable electrode arc welding for reducing spattering of molten metal by rapidly decreasing a welding current, immediately before the arc restrikes, upon detection of the squeeze of the droplet during the short circuit period.
2. Description of the Related Art
FIG. 5 shows electric-current and voltage waveforms and a droplet in consumable electrode arc welding in which the cycle of a short circuit period Ts and an arc period Ta is repeated. Specifically, Graph 5(A) shows a welding current Iw which passes through the consumable electrode (hereinafter called welding wire 1). Graph 5(B) shows a welding voltage Vw applied between a power supply chip and a base metal 2. Pictures 5(C)-5(E) show how a droplet 1a changes in shape.
During the short circuit period Ts between Time Point t1 and Time Point t3, a droplet 1a is at the tip of the welding wire 1, making a short-circuit with the base metal 2. In this period, as shown in Graph 5(A), the welding current Iw increases gradually, and as shown in Graph 5(B), the welding voltage Vw assumes a low value of a few volts due to the short-circuit situation. As shown in Picture 5(C), the droplet 1a makes contact with the base metal 2, thereby making the state of short-circuiting at. Time Point t1. Then, as shown in Picture 5(D), squeezing or constriction 1b develops in an upper portion of the droplet 1a due to an electromagnetic pinch caused by the welding current Iw passing through the droplet 1a. The squeezed droplet 1b becomes narrower rapidly, and at Time Point t3 as shown in Picture 5(E), the droplet 1a leaves the welding wire 1, into a molten pool 2a, which allows an arc 3 to restrike.
When the above squeezing phenomenon occurs, the short circuiting comes to an end within a very short period of time of a few tens through a few hundreds of microseconds (μs), whereby the arc 3 restrikes. This indicates that the squeezing phenomenon is a premonitory sign of the end of short circuiting. When the squeezed droplet 1b appears, the path for the welding current Iw becomes narrow at the squeezed portion, which increases the resistance r of the squeezed portion. The resistance r increases with the progress of the squeezing, i.e. as the squeezed portion becomes narrower. Therefore, it is possible to detect the squeezing phenomenon by detecting the change in resistance r between the welding wire 1 and the base metal 2 during the short circuit period Ts. The change in the resistance r can be calculated by its deferential value dr/dt=d(Vw/Iw)/dt. Since the squeeze period lasts only for a very short period, the change in the welding current Iw while the squeezing is present is very little, as seen from Graph 5(A). Therefore, the occurrence of the squeezing phenomenon can also be detected by the change in the welding voltage Vw, i.e. dVw/dt. In practice, the differential value of the resistance r or the welding voltage Vw is calculated in the short circuit period Ts, and it is checked if the differential value has reached a predetermined squeeze detection reference value Vtn. Although the following description assumes that the detection of the squeezed droplet is conducted on the basis of the differential value dVw/dt of the welding voltage, the detection may also be possible on the basis of the differential value dr/dt of the resistance, or other conventional methods.
Detection of arc recurrence (or arc restriking) at Time Point t3 is made by checking if the welding voltage Vw is no longer lower than a short-circuit determination value Vts. As readily seen, the period in which Vw<Vts is the short circuit period Ts, and the period in which Vw≧Vts is the arc period Ta. Before used for the short/arc detection, the welding voltage Vw is subjected to low-pass filtering to remove high frequency noises. Otherwise, erroneous detection may result due to the fact that the welding voltage Vw and the welding current Iw tend to greatly vary along with the change of the arc length which is caused by the change of the arc load during the arc period Ta. Such filtering may cause a delay of a few tens of microseconds in the short/arc detection. This delay does not give rise to any serious problems for typical consumable electrode arc welding, where the welding current Iw and the welding voltage Vw are controlled for both the short circuit period and arc period.
When the arc restrikes at Time Point t3, the welding current Iw decreases gradually as shown in Graph 5(A) whereas the welding voltage Vw assumes an arc voltage which is a few tens of volts as shown in Graph 5(B). During the arc period Ta, the tip of the welding wire 1 is melted to form a droplet 1a, and the base metal 2 is also melted. Generally in consumable electrode arc welding, a constant-voltage power source is used for realizing the optimum arc length. By consumable electrode arc welding accompanied with short circuiting, a low welding current average (hence a low wire feeding speed) leads to short-circuiting transfer welding, whereas a high welding current average leads to globular transfer welding.
If the current Ia is large at Time Point t3 when the arc restrikes in the welding, the pressure (arc force) from the arc 3 to the molten pool 2a becomes very large, and this produces a large amount of spatter. The amount of spatter increases essentially in proportion to the welding current Ia at the time of arc recurrence. If the spattering is to be reduced, it is necessary to decrease the welding current Ia at the time of arc recurrence. To achieve this, a number of proposals have been made for a squeezing detection control method of detecting the squeezing phenomenon and rapidly decreasing the welding current Iw thereby decreasing the welding current Ia at the time of arc recurrence. Hereinafter, these conventional techniques will be described.
FIG. 6 is a block diagram of a welding power source which operates on a conventional squeezing detection control method of the squeezed droplet. Note that FIG. 6 does not show blocks related to wire feeding.
A power source main circuit PM receives commercially available power (such as 200 V three-phase power) as an input, provides output controls (such as inverter control, thyristor phase control and so on) in accordance with error amplification signals Ea to be described later, and outputs output voltage Eo and welding current Iw. A parallel circuit consisting of a transistor TR and a resistor R is inserted in the power path. As described below, the transistor TR is turned off when a squeezed droplet is detected, which causes a rapid decrease in the welding current Iw passing through the parallel circuit. When a squeezed droplet is detected in a short-circuit state, the output from the power source main circuit PM is stopped, while the energy stored in a reactor in the power source main circuit PM is discharged. This decreases the welding current Iw. The speed of this decrease depends on the resistance of the short-circuit load and the inductance of the reactor. The resistor R is put at the illustrated position to increase the fall speed of the welding current when a squeezed droplet is detected. The resistance of the resistor R is a few tens of times the short-circuit load (which is about 0.01Ω). With this arrangement, the welding current Iw falls rapidly within about 100 μs when a squeezed droplet is detected. The welding wire 1 is supplied at a constant speed to generate an arc 3 between itself and the base metal 2.
The voltage detection circuit VD detects a welding voltage Vw and outputs a voltage detection signal Vd. A voltage differentiation circuit DV differentiates the voltage detection signal Vd and outputs a voltage differentiation signal Dv=dVw/dt. A voltage filter circuit VF filters the voltage detection signal Vd with a low-pass filter, thereby removing high frequency noise, and outputs a voltage filter signal Vf. A short circuit determination circuit SD compares the voltage filter signal Vf with a predetermined short-circuit determination value Vts and outputs a short circuit determination signal Sd which assumes High level during the short circuit period. A detection circuit of squeezed droplet ND outputs a squeeze detection signal Nd which assumes High level for a short period of time, at a time when the voltage differentiation signal Dv has reached a predetermined squeeze detection reference value Vtn (upon detection of a squeezed droplet) A flip-flop circuit FF outputs a squeeze detection period signal Tn which is set to Low level by the squeeze detection signal Nd and is reset to High level by a fall of the short circuit determination signal Sd (upon recurrence of the arc).
A drive circuit DR outputs a drive signal Dr which turns on the transistor TR when the squeeze detection period signal Tn is at High level. The squeeze detection period signal Tn assumes Low level during the squeeze period, upon detection of the squeezed droplet until the arc restrikes, during which the transistor TR is turned off, and the welding current Iw passes through the resistor R, thereby decreasing rapidly.
A rise period setting circuit TUR outputs a predetermined rise period setting signal Tur. A low squeeze current setting circuit IMR outputs a predetermined low squeeze current setting signal Imr. A high arc-current setting circuit IHR outputs a predetermined high arc-current setting signal Ihr. A detection current control circuit NIC receives the above-mentioned setting signals Tur, Imr, Ihr and the squeeze detection period signal Tn as inputs, and outputs a power source characteristic switching signal Sw and an electric-current setting signal Ir to be described later with reference to FIG. 7.
An output voltage setting circuit ER outputs a predetermined output voltage setting signal Er. An electric-current detection circuit ID detects the welding current Iw and outputs an electric-current detection signal Id. An output voltage detection circuit ED detects the output voltage Eo and outputs an output voltage detection signal Ed. A voltage error amplification circuit EV amplifies an error between the output voltage setting signal Er and the output voltage detection signal Ed, and outputs a voltage error amplification signal Ev. An electric-current error amplification circuit EI amplifies an error between the electric-current setting signal Ir and the electric-current detection signal Id, and output an electric-current error amplification signal Ei. A power characteristic switching circuit SW receives the power source characteristic switching signal Sw as an input, assumes Position “b” during the squeeze period and the rise period Tu to be described later with reference to FIG. 7, and outputs the electric-current error amplification signal Ei as an error amplification signal Ea, whereas during the other periods, it assumes Position “a” and outputs the voltage error amplification signal Ev as an error amplification signal Ea. Therefore, the period in which it assumes Position “a” is a constant-current characteristic period whereas the period in which it assumes Position “b” is a constant-voltage characteristic period.
FIG. 7 shows a timing chart of each signal in the welding power source described above with reference to FIG. 6. Graph 7(A) shows the welding current Iw, Graph 7(B) shows the welding voltage Vw, Graph 7(C) shows the short circuit determination signal Sd, Graph 7(D) shows the squeeze detection signal Nd, Graph 7(E) shows the voltage differentiation signal Dv, Graph 7(F) shows the squeeze detection period signal Tn, and Graph 7(G) shows the electric-current setting signal Ir.
During the short circuit period Ts from Time Point t1 to Time point t3, as shown in Graph 7(C), the short circuit determination signal Sd assumes High level but its rise and fall timings are delayed by Td because of the low-pass filtering operation described earlier. Therefore, the short circuit determination signal Sd assumes Low level at Time Point t4 which is a time point delayed from the arc recurrence Time Point t3 by the delay time Td. The length of the delay time Td is a few hundreds of microseconds as described above.
At Time Point t2, when squeezing appears at the droplet and the welding voltage Vw increases as shown in Graph 7(B), the voltage differentiation signal Dv=dVw/dt increases rapidly as shown in Graph 7(D) and reaches a predetermined squeeze detection reference value Vtn. As a result, the squeeze detection period signal Tn changes to assume Low level as shown in Graph 7(E). The squeeze detection period signal Tn continues to be at Low level until Time Point t4 when the short circuit determination signal Sd in Graph 7(C) assumes Low level. As shown in Graph 7(G), the electric-current setting signal Ir assumes the value of the high arc-current setting signal Ihr in FIG. 6 at Time Point t4 when the short circuit determination signal Sd in Graph 7(C) changes to Low level and during the rise period Tu which is determined by the rise period setting signal Tur in FIG. 6, whereas during the other period, it assumes the value of the low squeeze-current setting signal Imr. Simultaneously, though not illustrated in the chart, the power source characteristic switching signal Sw in FIG. 6 assumes High level to provide the constant-current characteristic during the period from Time Point t2 through Time Point t5 whereas it assumes Low level to provide the constant-voltage characteristic in the other periods.
During the period from Time Point t2 through Time Point t4 when the squeeze detection period signal Tn in Graph 7(E) assumes Low level, the transistor TR in FIG. 6 is turned off and as shown in Graph 7(A), the welding current Iw which falls rapidly from Time Point t2 is maintained at the low squeeze current Im which is the level set by the low squeeze-current setting signal Imr in Graph 7(G). At Time Point t3, the arc restrikes as shown in Graph 7(B), and the welding voltage Vw rises rapidly at Time Point t3. At this point of arc recurrence, as shown in Graph 7(A) the welding current Iw assumes the low squeeze current Im which is a few tens of amperes, whereby spattering is reduced. At Time Point t4, the electric-current setting signal Ir changes to the high arc-current setting signal Ihr as shown in Graph 7(G), and therefore, the welding current Iw increases as shown in Graph 7(A), to a high arc current Ih which is a value set by the high arc-current setting signal Ihr, and at Time Point t5 shifts to an arc current which is determined by the constant-voltage characteristic. With the above, as shown in Graph 7(D), the voltage differentiation signal Dv makes a rapid increase until Time Point t2, and then makes a rapid decrease from Time Point t2 as the welding current Iw decreases rapidly. Then, when the arc restrikes, the signal makes a rapid increase again, following the rapid rise of the welding voltage Vw. After Time Point t3, the signal assumes the value zero essentially, since the welding voltage Vw does not change very much.
According to the conventional art, as explained above, spattering can be decreased by controlling the welding current to the low squeeze current Im when the arc restrikes (See JP-A-2006-26718).
In the conventional art, however, the fall of the short circuit determination signal Sd (Time Point t4) is delayed by the time Td as shown in Graph 7(C) from the recurrence of the arc (Time Point t3). This is because, as already described, the welding voltage Vw is passed through a low-pass filter in order to eliminate misdetection of a short circuit. Because of this delay time Td, as shown in Graph 7(A), the welding current Iw does not increase but stays at a low value when the arc restrikes at Time Point t3. Further, the delay time Td is a fixed length of period determined by the low-pass filter setting, and the value, which is a few tens of microseconds, is not a negligible length.
In short-circuit transfer welding, which uses a relatively low current range, the delay time Td does not pose a major problem in welding stability. However, in middle-current through large-current operation such as welding in which short-circuit transfer welding and globular transfer welding are both present, or in globular transfer welding, or in spray transfer welding, the delay time Td often affects the welding stability since short-circuiting often occurs irregularly and therefore there is a wide range of variation in the droplet size when the short circuit occurs. The delay time also causes a wide range of differences in the time course of the squeezed phenomenon and in squeeze detection accuracy. As a result, a squeezed droplet is not detected sometimes until toward the end of the squeezed phenomenon, and in such a case, the arc restrikes in an extremely short time (less than a few tens of microseconds) after the squeezed droplet is detected.
FIG. 8 shows a timing chart, corresponding to those in FIG. 7, which depict a case where the arc restrikes immediately after a squeezed droplet is detected. Graph 8(A) through Graph 8(G) show different patterns of the same signals as shown in FIG. 7.
As a squeezed portion grows narrower at Time Point t2, the welding voltage Vw increases as shown in Graph 8(B). At the same time, the voltage differentiation signal Dv increases rapidly as shown in Graph 8(D) to reach the squeeze detection reference value Vtn, while as shown in Graph 8(E), the squeeze detection signal Nd assumes High level for a short period of time. In response to this, as shown in Graph 8(F), the squeeze detection period signal Tn changes to assume Low level, switching off the transistor TR. Thus, the welding current Iw falls rapidly as shown in Graph 8(A).
When the arc restrikes at Time Point t3, immediately after Time Point t2, the welding voltage Vw rises rapidly as shown in Graph 8(B). However, as shown in Graph 8(C), the short circuit determination signal Sd falls with a delay by Td due to the low-pass filter, and the signal continues to be at High level until Time Point t4. During this period, the squeeze detection period signal Tn in Graph 8(F) continues to be at Low level, and therefore the transistor TR continues to be in the OFF state, and as shown in Graph 8(A), the welding current Iw assumes the low squeeze current Im. At Time Point t4 when the delay time Td has lapsed as shown in Graph 8(C), the short circuit determination signal Sd changes to Low level (arc state). In response to this, as shown in Graph 8(E), the squeeze detection period signal Tn assumes High level to turn on the transistor TR. Simultaneously, as shown in Graph 8(F), the electric-current setting signal Ir assumes the high arc-current setting value Ihr, and thus the welding current Iw begins to increase as shown in Graph 8(A).
In middle-to-high current range welding, wire feeding speed is greater than in low current range welding, and vibration of the molten pool is also greater. If the welding current Iw is low during the period from Time Point t3 (when the arc restrikes) to Time Point t5, the melting speed of the wire becomes slower than the feeding speed, and the arc length becomes shorter. In this state, when the molten pool vibrates, short circuiting occurs at Time Point t5. This short circuiting takes place in a state where the wire tip is not melted (without a droplet).
When the short circuiting occurs at Time Point t5, the welding voltage Vw drops to a low value, as shown in Graph 8(B), and the welding current Iw, as shown in Graph 8(A), increases with the short-circuit load, while the short circuit determination signal Sd assumes High level with a slight delay, as shown in Graph 8(C). Since the wire tip is not melted in this short circuiting, the short circuiting does not come to an end even at Time Point t6. Thus, in order to forcefully terminate the short-circuit, the welding current Iw is increased as shown in Graph 8(A), to an extraordinary great value (“short-circuit termination value”). As a result, the wire melts at Time Point t7 and the arc restrikes. In this process, no squeezed droplet is formed since it is the solid wire that is broken. After Time Point t6, the system is in a short-circuit termination process, and in this process, the squeeze detection signal Nd shown in Graph 8(E) is not produced even when the voltage differentiation signal Dv has reached the squeeze detection reference value Vtn as shown in Graph 8(D).
In conducting the above-described short-circuit termination, it has been found that the large amount of current passing through the wire causes excessive spattering, and leads to unstable welding condition. Such an appropriate welding condition incurs irregular occurrence of short-circuiting and significantly degrades the quality of welding.